The present invention refers to tubular electrodes for lead-acid storage batteries. Tubular electrodes consist of a plurality of metallic rods or cores arranged parallel to and at a distance from each other, which cores are conductively connected with each other. Each of the metallic cores is surrounded by and in contact with active material which, in turn, is surrounded by a tubular sheath.
The rod sheaths are electrically insulating. The material of the sheaths is resistant to the electrolyte and to the attacks to which it is exposed in the battery cell during operation, especially oxidation due to nascent oxygen. The sheaths must be easily permeable by the electrolyte, produce as little electric resistance as possible, and prevent the active material from losing contact with the conductive core.
At one time the sheaths were made of slit hard rubber but have later come to comprise woven or braided materials of various kinds of impregnated threads or of thermoplastic foil or the like. The replacement of the hard rubber tubes with woven material constitutes a big step forward with respect to durability, capacity and effectiveness. It has been proposed that the sheaths comprise braided or woven threads of plastic fiber, for example, a polyester of ethylene glycol and terephthalic acid. Sheaths comprising a glass fiber weave impregnated with some plastic that is resistant under these conditions, for example phenol resin, have come into use with some success.
Glass of suitable quality has been found especially usable for the fibers in the preparation of the woven or braided sheaths in that the glass does not stretch, or at least only insignificantly. This keeps the active material in good contact with the conductive rods or cores despite the tendancy of the active material to swell during electrical discharge of the battery. If a braided tube is under pressure it will increase in diameter and at the same time become shorter in length. For that reason, axial warps or stem threads have been used to bind the weave or braid.
It has been found difficult to bind together the weave or braid of the sheaths due to the smooth surface of the glass fibers. It is not permissible for the glass threads in the sheaths to slide in relation to each other and in that way to create an irregular weave structure whereby in some places the distance between the threads may become so large that active material can force itself through and thus lose contact with the electrode. To prevent the appearance of excessively large openings in the sheath weave or braid when the glass fiber yarn is used, it has been proposed that stem threads of thermoplastic material, for example, polyethylene or polypropylene be provided which extend in the axial direction of the sheath. These stem threads would be welded to the outer usually multifiber glass weave threads by a suitable heat treatment.
Braided or knitted sheaths have also been proposed. These sheaths consist of thermoplastic monofilament threads which are welded together by a heat treatment and thereby receive the necessary rigidity for the subsequent application to the electrode skeleton.
All the electrode sheaths enumerated hereabove are subject to certain disadvantages, which have resulted in some of the constructions having been abandoned. It has also been found that modern types of glass compounded for the purpose are preferably to the plastic. The glass is also considerably more inexpensive than corresponding threads of other material suitable for the purpose.
As previously mentioned, however, the glass threads in the sheath weave must be fixed, i.e., locked to each other, so that they cannot shift in relation to each other. When woven or braided threads of thermoplastic material are employed, it has been found that the plastic material will shrink and thus shorten the length of the sheath during the heat treatment.
A braided sheath having axial stem threads must be braided to an exact diameter such as with the help of a mandrel, contrary to a braid without stem threads which is able to change in diameter with changes in length of the sheath. In a continuous heat treating process involved braided sheaths with thermoplastic stem threads, the braiding must be tensioned to prevent shrinkage thereof and a resulting increase of the sheath diameter, as well as to advance the sheath along the mandrel. However, if the steam threads are merely thermoplastic fibers which are softened or melted in order to bind the glass fibers, the tensioning of the sheath would cause the sheath to the stretched and elongated and the diameter thereof would become smaller.
In Sundberg U.S. Pat. Nos. 3,801,399 and 3,694,265, stem threads are used which comprise two different fibers, one glass and one thermoplastic. The glass fibers prevent a changing in length of the sheath during the manufacturing process. However, while this method provides sheaths with a very uniform diameter, it is very difficult to produce uniform internal stressing in the stem threads of glass. That is, during the braiding process, the axial threads tend to twist and untwist, resulting in the formation of internal stressing of the glass fibers, which can be non-uniform from one fiber to the next. Subsequent to the heating process when the tension is released, the various stressed glass fibers may tend to contract different amounts, thereby tending to deform the sheath with bends or twists. For instance, if the sheath is rectangular and the shorter stem thread is located in a corner of the sheath, it will result in a twisted sheath.
It is an object of the present invention to provide a novel electrode sheath.
It is another object of the invention to provide a novel electrode sheath which is not subject to bending or twisting during manufacture.
It is an additional object of the invention to provide a novel electrode sheath in which the lengths of axial stem threads are essentially uniform.